The measurement of the motion of gas, particularly that of air, is important for a wide range of applications. Methods for the control of combustion and other gas processing require the measurement of mass flow rate, which implies a measurement of velocity. Similarly, the control of aircraft requires accurate measurement of the angle-of-attack and sideslip. The measurement of velocity flow fields around aircraft, cars, electronic components, etc., is an important factor in developing optimum design. Studies of turbulence require an accurate measurement of velocity fields, as does the understanding of mixing processes, bursting, vorticity, and other phenomena associated with the unstable motion of fluids. Understanding these mechanisms is critical for the development of high-performance aircraft, compressors, turbines, engines, low-drag vehicles, and effective air cooling systems. The accurate real-time measurement of velocity may lead to the implementation of advanced strategies for the control of turbulent and unstable phenomena.
In many cases, it is important to acquire velocity data without interfering with the flow field. This is particularly true for unsteady and turbulent flows where an interfering measurement may significantly perturb the flow field. Nonintrusive measurements are also important in high-speed flows where shockwave structure may form around an intrusive probe, obscuring a true measurement of the flow velocity. In unsteady flows, the simultaneous measurement of velocity at multiple points is preferred.
Various approaches have been developed for the nonintrusive measurement of velocity. Perhaps the most common one is based on particle scattering and is called "Laser Doppler Velocimetry (LDV)" (see Reynolds et al. U.S. Pat. No. 4,715,707). This approach generates single point measurements which can only occur when a particle passes through a small volume defined by the intersection of two laser beams. Thus, it is not a field measurement approach and requires that the flow either be seeded or have a relatively high density of naturally occurring particles.
"Particle Imaging Velocimetry (PIV)" (R. Adrian, Int. J. Heat & Fluid Flow, Vol. 7, #2, June 1986, p. 127) is another approach to generating velocity field measurements. In this case, a high-intensity sheet of light illuminates many particles in a plane and this scattering is recorded by a camera. A short time later the particles are illuminated again, and their new location is again recorded. The displacement of each particle is then a measure of the local flow field velocity.
Particles can also be used for global velocimetry by observing the brightness of the scattering as a function of the Doppler shift when the scattered light is passed through a spectral filter with a linear variation of the transmission as a function of frequency (Komine U.S. Pat. No. 4,919,536). One can also use optical heterodyne (Breen U.S. Pat. No. 4,822,264), interferometric (German DT U.S. Pat. No. 2,500,376; Domey et al. U.S. Pat. No. 4,822,164; or Rizzo U.S. Pat. No. 3,825,346) or spectral analysis (Woodfield U.S. Pat. No. 4,585,341) to observe Doppler shifts from individual particles or volume elements containing particles. All these approaches require that the flow be seeded or have naturally occurring particles.
Small particles have been shown to be good indicators of flow motion for low-speed flows. In high-speed compressible flows, however, the particles cannot track the motion unless they are so small that they are no longer useful for LDV, PIV, or other particle related measurements. Furthermore, unless the particle density is extremely high, there are gaps in the flow field velocity measurement due to the absence of particles. On the other hand, if the particle density is too high, particle methods are no longer viable since, in the cases which require single particle observations, more than one particle occupies the scattering volume, and in the planar imaging cases, one loses the ability to determine which particles seen in the second image correlate with the original particles in the first image. In the linear filter approach, high particle densities lead to secondary scattering; so the observed brightness of the particle is not a true measure of the velocity.
Another approach to the nonintrusive measurement of flow velocity relies on narrow linewidth laser-induced fluorescence. In this case, the Doppler shift of a narrow linewidth atomic or molecular transition is observed by using a narrow linewidth laser source. To date, no appropriate absorption lines have been found in naturally occurring air, so these approaches required that the flow fields be seeded with foreign vapors including sodium (R. Miles, Physics of Fluids 18, p. 751, 1975), iodine (J. McDaniel, B. Hiller, and R.K. Hanson, Optics Letters, 8, #1, p. 51, Jan. 1983), or nitric oxide (P.H. Paul, M.P. Lee, and R.K. Hanson, Optics Letters 14, #9, p. 417, May 1989).
Velocity can also be measured by observing the Doppler shift associated with direct Rayleigh scattering from molecules seen through a sharp cutoff, narrow linewidth optical filter (R. Miles, Appl. Phys. B 51, #1, p. 1, 1990). This approach does not require particle or molecular seeding and leads to field measurements of velocity. The accuracy of the measurement is limited by the low Rayleigh scattering cross section and it is difficult to follow particular fluid elements to observe velocity profiles and turbulent structure.
Various approaches to flow tagging have been attempted including simple heating of a foreign material (J. Sell and R. Cattolica, Appl. Optics 25, #9, p. 1420, May 1986), and laserinduced dissociation of water (L. Boedeker, Opt. Lett. 14, #10, p. 473, May 1989). In liquids, the photochromic effect has been exploited by using an argon-fluoride laser to write colored lines into a photochromic compound in kerosene (R. Falco and L. Chu, SPIE Vol. 814, "Photomeohanics and Speckle Metrology", p 706, 1987). Current work is underway to expand this technique to air flows by putting the photochromic compound into a mist and tracking the motion of the mist suspended in the air.